The present invention relates generally to methods for hot working metals and alloys for attaining selected microstructure, and more particularly to a method for producing homogeneous wrought microstructure during equal channel angular extrusion (ECAE) of difficult-to-work high temperature alloys.
Wrought processing of metals often involves hot working during initial (ingot breakdown) and intermediate fabrication steps. Typical hot working techniques include hot extrusion and various types of hot forging (cogging, pancake forging, etc.)
Conventional ECAE (see Segal et al, U.S. Pat. No. 5,400,633; Segal, "Working of Metals by Simple Shear Deformation Process," Proceedings Fifth International Aluminum Technology Seminar, Vol 2, 403-6 (1992); Segal et al, "Plastic Working of Metals by Simple Shear," Russ Metall, Vol 1, 99-105 (1981)) can be used for primary breakdown and secondary hot working of an alloy ingot. Large deformations can be imparted without change in ingot cross section, which permits small ingots to be melted to obtain a given semi-finished product size. ECAE is especially useful for materials prone to macrosegregation during casting of large ingots. Other advantages include moderate working pressures (compared to extrusion through converging conical dies) and control of crystallographic and mechanical texture during multi-pass ECAE by selective rotation of the workpiece between passes.
Although ECAE is useful for imparting large deformations to a variety of alloys, because of the simple shear nature of the deformation during ECAE, the deformation is confined to a narrow-shear zone. Thus, materials that exhibit flow softening are prone to flow localization in shear. In ECAE, formation of a shear band or shear crack results in the development of grossly nonuniform flow. Eventually, the final extruded billet exhibits a periodic series of shear bands (or shear cracks) and undeformed zones. Development of shear bands or shear cracks leads to undesirable, grossly nonuniform microstructures. Thus, the conventional (Segal et al) ECAE process may not be useful for materials that exhibit significant flow softening during hot working.
The invention solves or substantially reduces in critical importance problems with conventional ECAE by providing a method for ingot breakdown or secondary hot working via ECAE of difficult-to-work alloys that exhibit extensive flow softening, resulting in homogeneous deformation and development of uniform microstructure in alloys such as conventional titanium; nickel, iron, and cobalt base superalloys; and intermetallic alloys in the cast or wrought condition, by subjecting the alloy to an increment of upset deformation prior to shear flow in ECAE.
It is therefore a principal object of the invention to provide a hot-working method for metals and alloys.
It is another object of the invention to provide a method for hot working metals and alloys for preselected microstructure in the hot worked product.
It is a further object of the invention to provide a method for hot working difficult-to-work high temperature alloys to produce homogeneous wrought microstructure.
It is another object of the invention to provide a method for hot working high temperature aerospace alloys to produce uniform deformation and uniformly wrought microstructure by ECAE.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.